For starting off, gotta go with the cube, though green slime is a close second.
For tough monsters, you have to go with the beholder. I remember when I was a young lad, hearing about the soon to be televised D&D Cartoon - and what excited my young little mind the most about that, for some strange reason, was being able to see a Beholder in action. In the end, I was rather disappointed with the series, though I still watched it - and there eventually WAS a beholder, but they way they put it in was just terribly disappointing. I only vaguely recall it now.
I liked seeing beholders in the D&D Movie, though they obviously were not played right.
Speaking of which, I wonder if there really will be a second one - seems that the buzz on that has vanished...